It is known to form aluminum and aluminum alloy coatings upon steel sheet or strip by hot-dip coating. The processes are many, some comprising a variation of the well known Sendzimir process for galvanizing carbon steel strip. The purpose of providing the aluminum or aluminum alloy coating on the strip is to protect the steel from corrosion. Hence, any hot-dip coating process seeks to minimize uncoated portions of the strip including pinhole bare spots. Moreover, the coating must be tightly adhered to the surface of the steel so that it does not separate during fabrication or use.
As used herein, the terms "sheet" and "strip" are used interchangeably and are meant to include flat rolled products including plate, sheet and strip.
Hot-dip aluminum coated steel exhibits a high degree corrosion resistance to salt and other corrosive atmospheres. Hence, it finds use in various applications including automotive exhaust systems. In recent years, automotive combustion gases have increased in temperature making them even more corrosive. For this reason, there has become a need to increase the high temperature oxidation resistance and salt corrosion resistance by replacing aluminum coated low carbon or low alloy steels with chromium-containing steels, preferably, high formability, aluminum coated stainless steels. Other applications may include power plants and high temperature uses where exposure to severe corrosive environments exist.
While the patent literature contains references to hot-dip coated stainless steels, see for example, U.S. Pat. Nos. 3,378,359; 3,907,611; 3,925,579; 4,079,157; 4,150,178; 4,601,999; and 4,883,723, it is well known that these are more difficult to coat than carbon steels. The ferritic grades of chromium stainless steels are known to be even more difficult than the austenitic grades. It is known that it is especially difficult to coat stainless steels with aluminum-silicon alloys with more than 0.5% silicon. The pure aluminum (ASTM A 463-88 Type 2 coatings) forms a thicker alloy layer than one containing 5% to 11% silicon (ASTM A 463-88 Type 1 coatings). Because the iron-aluminum alloy layer that forms at the surface of the steel strip is very hard and brittle, a thick alloy layer makes the formability of the coated strip even worse. For this reason, Type 1 coatings are preferable, particularly in difficult forming applications.
In Kilbane et al. U.S. Pat. No. 4,883,723, there is disclosed a process for hot-dip coating ferritic stainless steels containing at least 6% chromium and less than 3% nickel with a Type 2 coating. The surface of the steel is cleaned by pretreating to remove oil, dirt, oxides and the like, and then is heated to a temperature near or slightly above the melting point of the coating metal, at least about 677.degree. C. (1232.6.degree. F.), and then is protected in an atmosphere containing at least about 95% by volume hydrogen and a dew point of no more than+40.degree. F. (3.degree. C.). The Kilbane et al. process discloses that it is not applicable to Type 1 alloy coatings.
Other processes for making premium products involve preliminary plating of the stainless steel strip with iron, nickel or iron plus boron to prevent oxidation of the chromium. With these processes, both Type 1 and Type 2 coatings can be applied. While the coated strip has excellent properties, this process is very expensive due to higher capital costs, additional process steps and slower processing.